1. Field of the Invention
The present invention relates to an optical branching device, and more particularly, to an optical branching device for branching light in multimode transmitted in a planar waveguide.
2. Description of the Related Art
A Y-shaped branching waveguide, in which a waveguide is branched into two, has been known as a branching system of a planar multimode optical waveguide. A mode division system has been also known for coupling the light radiated from a bending portion of an optical fiber with an input end of another optical fiber as a branching system of a multimode optical fiber.
Further as shown in FIG. 10, the mode division system has been applied to the planar multimode optical waveguide. In an optical branching device as shown in FIG. 10, light is radiated from a core U1 to a cladding V2 by bending a portion of a main waveguide 1 and is coupled with a branching waveguide 2.
However, the conventional optical branching devices described above have several problems.
The Y-shaped branching waveguide has problems in transmission efficiency, because the light is easily lost during its transmission. That is, when a branching angle of the waveguide is large, an incident angle of the light becomes smaller than a critical angle of the waveguide at a branching point and radiation loss emerges. When the branching angle is small, scattering loss emerges due to dull branch shape and transmission loss also emerges, because each of the branching waveguides is elongated in order to be separated enough from each other. As a result, transmission efficiency of the Y-shaped branching waveguide needs to be improved.
The optical branching waveguide 400 as shown in FIG. 10 applying the mode division system to the planar multimode optical waveguide compensates for some of the shortcomings of the Y-shaped branching device. The optical waveguide 400 can branch light with a little loss, because it is designed to use radiated light which is originally to be lost in other devices. Further, the main waveguide 1 of the optical waveguide 400 can obtain a large branching angle and shorten the length of the waveguide 400, because the main waveguide 1 can be bent sharply. However, such structure incurs other problems. The light radiated from the main waveguide 1 becomes scattered and expands to the cladding V2 due to the rugged surface unintentionally formed on the sides of the core U1. As a result, a little light radiated from the main waveguide 1 is caught by an input end 5 of the branching waveguide 2 and thus transmission loss becomes large.